Method and apparatus for carrying digital data as analog video

ABSTRACT

An interface, e.g., a modem-like interface for converting a digital information signal into an analog signal for use within an analog television studio infrastructure. Namely, the present invention employs a novel modulator that conveys compressed video data into analog video lines by creating an analog “video” signal that contains video gray-scale levels which correspond to digital data values, instead of containing an image.

This application claims the benefit of U.S. Provisional Application No.60/122,817 filed on Mar. 4, 1999, which is herein incorporated byreference.

This invention was made with U.S. government support under contractnumber NIST 70NANB5H1174. The U.S. government has certain rights in thisinvention.

The invention relates to communications systems generally and, moreparticularly, the invention relates to an apparatus and method fordistributing digital information, such as MPEG-like data, over an analogsystem having an analog infrastructure.

BACKGROUND OF THE DISCLOSURE

Existing television studios or TV plants, by and large, use analog videosignals carried over 75-ohm coax to convey signals around their plant.Each television studio has a large investment in its infrastructure suchas routing “grids” which allow signal routing to be changed as neededwithin the plant.

With the proliferation of digital television signals, these costlyexisting analog infrastructures are incompatible in dealing with digitalinformation. A small fraction of TV plants have converted over to thenewer “SDI” 270 Mb/s digital system (SMPTE 259M) for conveyinguncompressed digital standard definition video, and additional standardsare emerging (e.g., SMPTE 305M) that define how to use thisinfrastructure to convey all kinds of other data, especially compressedvideo. The reluctance or slow pace of conversion to an all-digitalinfrastructure is due substantially to cost and the large investment inthe analog infrastructures. As such, the majority of current TV stationsstill operate all-analog infrastructures. Thus, it would be veryadvantageous if digital information such as Advanced Television SystemsCommittee (ATSC) signals can be routed within the existing analoginfrastructures.

Therefore, there is a need in the art for an apparatus and method fordistributing digital information over an analog system having an analoginfrastructure.

SUMMARY OF THE INVENTION

An embodiment of the present invention employs an interface, e.g., amodem-like interface for converting a digital information signal into ananalog signal for use within an analog television studio infrastructure.Specifically, digital data is converted into a National TelevisionSystems Committee (NTSC)-like waveform that uses the active videoportion of the signal to carry the digital data. Namely, the presentinvention employs a novel modulator that conveys compressed video datainto analog video lines by creating an analog “video” signal thatcontains video gray-scale levels which correspond to digital datavalues, instead of containing an image. The data is represented in amanner that is robust and is capable of withstanding the distortionsthat it may encounter on its way through various media, includingfrequency rolloff, phase distortion, proc amp blanking, clipping, andother video waveform distortion effects.

BRIEF DESCRIPTION OF THE DRAWINGS

The teachings of the present invention can be readily understood byconsidering the following detailed description in conjunction with theaccompanying drawings, in which:

FIG. 1 depicts a graphical representation of a digital informationsignal being represented as an analog video signal;

FIG. 2 depicts a block diagram of a communications system fordistributing digital information over an analog infrastructure;

FIG. 3 depicts a block diagram of a modulator of the invention;

FIG. 4 depicts a block diagram of a demodulator of the invention; and

FIG. 5 depicts a graphical representation of modulation spectracorresponding to multiple modulations.

To facilitate understanding, identical reference numerals have beenused, where possible, to designate identical elements that are common tothe figures.

DETAILED DESCRIPTION

FIG. 2 depicts a block diagram of a communications system 200 of thepresent invention for distributing digital information, e.g., ATSC orother Moving Picture Experts Group (MPEG)-like data over an analoginfrastructure 220. The communications system 200 comprises a modulator210 and a demodulator 230.

It should be noted that FIG. 2 also depicts one embodiment of thepresent invention where the modulator 210 and demodulator 230 areimplemented within general purpose computers. In such embodiment, ageneral purpose computer may comprise a modulator 210 (or demodulator230), a processor (CPU) 212 (or 232), a memory 214 (or 234), e.g.,random access memory (RAM), and various input/output devices 216 (or236), (e.g., a keyboard, a mouse, an audio recorder, a camera, acamcorder, a video monitor, any number of imaging devices or storagedevices, including but not limited to, a tape drive, a floppy drive, ahard disk drive or a compact disk drive).

It should be understood that the modulator 210 (or demodulator 230) canbe a physical device that is coupled to the CPU through a communicationchannel. Alternatively, the modulator 210 (or demodulator 230) can berepresented by one or more software applications (or even a combinationof software and hardware, e.g., using application specific integratedcircuits (ASIC)), where the software is loaded from a storage medium,(e.g., a magnetic or optical drive or diskette) and operated by the CPUin the memory of the computer. As such, the modulator and demodulator(including associated data structures) of the present invention can bestored on a computer readable medium, e.g., RAM memory, magnetic oroptical drive or diskette and the like.

Specifically, in one embodiment the communications system 200 employsthe modulator 210 for converting an ATSC signal into a NTSC signal,which is then stored, distributed, or routed through an existing analogNTSC infrastructure 220. When the analog NTSC signal is routed to itsdestination, the demodulator 230 demodulates the analog NTSC signal backinto the original ATSC signal.

In operation, the digital stream on path 205 goes into the modulator 210that creates a waveform as illustrated below in FIG. 1, and thisNTSC-like signal may be carried throughout the analog NTSCinfrastructure 220. At any destination point that receives the NTSC-likeor NTSC-compliant signal, a demodulator 230 is employed to extract andoutput the original digital data.

To illustrate, FIG. 1 is a graphical representation of digital data(e.g., ATSC data, MPEG-like data, Digital Video Broadcasting (DVB) data,digital audio data, meta data, control data and the like) beingrepresented by an analog video signal or, more particularly, within the“active video portion” of the video line. Namely, the “active videoportion” is typically where the picture of an NTSC signal will occupy.Thus, although the analog waveform carrying the digital data isNTSC-compliant, any attempts to actually view the active video portionof this NTSC-compliant signal will likely produce an image thatresembles noise. However, from the perspective of the analoginfrastructure 220, such NTSC-compliant signals can be stored,distributed, or routed like any other NTSC-compliant signals thatactually carry images in the active video portion of the signal. Thus,the desired result is achieved where digital data can be carried withinan analog infrastructure, thereby allowing TV studios to continue use oftheir existing analog systems.

Specifically, FIG. 1 shows multiple lines (overlaid) each with 32symbols 100 of four levels 110 each. It should be noted that theresulting video signal 100 carrying the digital information is precededby a sync pulse 120 and a color subcarrier or burst (a high frequencysine wave) 130, which are NTSC timing system signals.

Namely, the example illustrates each TV line with thirty-two (32)symbols having four (4) levels for each symbol. Since four levels can berepresented by two bits, each line represents 64 bits. With 483 activelines per frame and 30 frames per second, this gives 14,490 active linesper second, or 927 kbits/sec. This rate may be too low to support ATSC,but fortunately the NTSC signal can support many more symbols per lineas well as more levels per symbol.

Thus, if a rate of 20 Mbits/sec is desired, then one would fit 20Mbits/sec into 14,490 lines/sec, thereby needing to pack 1,381 bits intoeach line. Given a 4.2 Mhz bandwidth, one can fit 450 symbols into a 53μSec active line time (one symbol per half-cycle of 4.2 MHz), therebyimplying 1381/450, or that slightly more than 3 bits per symbol isrequired.

Using 16 levels (4 bits) per symbol and choosing the NTSC subcarrier ashalf the symbol clock, the present apparatus will provide 379 symbolsper line, thereby achieving (4 bits/symbol)*(379 symbols/line)*(14,490lines/sec)=21.966840 Mbits/sec. It should be noted that the foregoing isan example only, and the present invention is not so limited. In thepreferred embodiment, the present apparatus should relate the symbolclock to the digital signal (e.g., NTSC) subcarrier by a factor which isthe ratio of integers, e.g., 2:1 in the above case.

Choosing the NTSC subcarrier as half the symbol clock is one of theimportant aspects of the present invention. Namely, the color subcarrieror burst 130 can be used as a reference for the symbol clock, therebymaking the present apparatus more robust. This allows the symbol clockto be quickly acquired and locked by the demodulator 230, since thecolor burst signal 130 is typically very stable and its relationship tothe data 100 is known. This allows the demodulator 230 to quicklyacquire the symbol clock and begin decoding data.

FIG. 3 depicts a block diagram of the modulator 210 of the presentinvention. The modulator 210 comprises a controller 310, a forward errorcorrection (FEC) module 320, a buffer 330 (e.g., First-In-First-Out(FIFO)), a modulator (or mixer) 340 and an adder 350.

In operation, digital information is received into the modulator 210 onpath 305. The digital information undergoes a series of processing stepswithin the forward error correction module 320 that is intended to allowrecovery of the data signal if errors occur. For example, the forwarderror correction module 320 may incorporate a data randomizer, one ormore encoders (e.g., Reed-Solomon encoder or Trellis encoder), and adata interleaver. The data randomizer (not shown) can be tasked withrandomizing the incoming digital data payload, e.g., MPEG transportpackets.

The encoders (not shown) are employed to apply one or more forward errorcorrection processes to the incoming digital data signal. FEC processesare error correction schemes that are designed to correct bit errorsthat may occur during transmission, e.g., noise, multipath propagation,transmitter non-linearities, and signal fades. The present invention isnot limited to a particular FEC process.

The data interleaver (not shown) is employed to scramble the sequentialorder of the data stream and to disperse the data packets throughouttime, e.g., over a period of time such as several milliseconds. Thepurpose of the data interleaver is to disperse the data packets in orderto minimize the transmitted signal's vulnerability to burst typeinterference. Namely, the data interleaver assembles tiny fragments ofscrambled data packets into new data packets, where the reconstituteddata packets have the same length as the original data packets. Thus, abrief noise burst will only cause the loss of some of the data in thestream for any given period of time. It should be noted that the variousfunctions performed by the forward error correction module 320 can beimplemented within the modulator 210 or outside of the modulator 210 ina pre-processing module.

Returning to FIG. 3, the “error corrected” signal from the forward errorcorrection module 320 is stored in a buffer (FIFO) 330. The storedbitstream is then sent from the FIFO under the control of controller 310to the modulator or mixer 340, where the digital ATSC signal ismodulated into an NTSC analog signal.

It should be noted that the FIFO will periodically stop sending data tothe modulator 340 under the control of the controller 310. Namely, dueto the effect of horizontal and vertical blanking in an NTSC compliantsignal, the modulator 340 must pause to allow the sync and burstportions of the NTSC compliant signal to be devoid of any videoinformation. Thus, when the modulator 340 is paused, the controller willforward or supply the sync and burst portions of the NTSC compliantsignal to the adder 350, thereby forming an NTSC compliant signal onpath 355. As such, the controller 310 is illustrated as receiving areference signal or a Genlock input on path 307, e.g., a black burstsignal, a color burst signal, a sync signal, a subcarrier signal and thelike. In fact, alternatively, the controller 310 can be implemented suchthat these reference signals are generated by the controller 310 itself.

FIG. 4 depicts a block diagram of the demodulator 230 of the presentinvention. The demodulator 230 comprises a symbol clock recovery module410, an inverse forward error correction module 420, a buffer 430 (e.g.,First-In-First-Out (FIFO)) and a demodulator (or demixer) 440. Thesymbol clock recovery module 410 is tasked with acquiring the timingsignals, e.g., the sync and burst signals, thereby acquiring the symbolclock as discussed above due to the known relationship of the symbolclock and the burst signal. In brief, the inverse forward errorcorrection module 420, and demodulator (or demixer) 440 are simplyperforming the inverse functions performed by the forward errorcorrection module 320, and modulator (or mixer) 340. Finally, thedigital information is stored in the buffer 430 and is clocked out inaccordance with control signal on path 435.

Alternative embodiments of the present invention are now describedbelow. First, it should be noted that other data modulation schemescurrently in use can be adapted to the present invention. For example,Quadrature Amplitude Modulation (QAM) may be used, with a carrierfrequency chosen to keep the lower end of the QAM spectrum from foldingaround DC. Additionally, it should be noted that either using morelevels and/or more bandwidth will allow for a higher data rate.

Second, a portion of the “Analog NTSC Infrastructure” 220 of FIG. 2 mayinclude a VTR so that the digital signal may be recorded on an analogVTR and played back through the demodulator at a later time. Thus, thedata modulation scheme may be modified to allow use with certain VCRswhich have reduced bandwidth. For example, some VCRs (e.g., VHS) extractsome of the video spectrum as “chroma” but do not reconstruct it wellenough for this purpose, so the data spectrum may be designed to avoidfrequencies above 2.5 or 3.0 MHz.

Third, those skilled in the art will recognize that the present systemis described with reference to NTSC video, but it can be adapted forPhase Alternation by Line (PAL) or (Sequential Technique and MemoryStorage) SECAM video.

Fourth, special reference data patterns may be multiplexed along withthe payload data at certain times to provide a measure of the channelquality. Furthermore, special waveforms (not just data patterns) may besent at regular intervals to allow automatic gain control (AGC)circuitry in the processing chain to maintain appropriate gain levelsfor the data pattern.

Fifth, certain portions of the data stream may be reserved for specialcontrol data. These may be partitioned as just a reserved part of theregular data stream, or a certain section of the video waveform e.g.,one or more lines may be reserved for this use. Furthermore, a differenttype of modulator (modem) scheme may be used on certain portions of thevideo waveform, e.g., a scheme that is more robust (lower symbol rateand/or fewer bits/symbol) to allow communication and control even incases where the channel quality is too poor to convey the higher-speeddata. For example, a two-way architecture may be devised using two (2)modulator/demodulator pairs and a pair of video connections (e.g., anoptional pair of modulator/demodulator 218 and 238 is shown in FIG. 2 indashed boxes). This allows for the modulators to “communicate with eachother” and determine, among other things, the quality of the channel, orto convey control data and to verify that the connections are made eventhough the high-speed data may not be decodable. This would allow forinstance debugging of a poor connection.

Sixth, as indicated above, it is possible to lock the symbol clock tothe subcarrier frequency by the ratio of two integers. The leading edgeof the horizontal sync could additionally provide phasing information toindicate the location of the first symbol, but due to the practice ofreplacing sync and burst signals along the processing chain, thisrelationship may be lost, so this technique may not be appropriate insome applications. However, the color burst signal is well-known to be avery clean and stable frequency reference, and so that it can beemployed as a symbol clock frequency reference, but not as a symbolphase reference, which may be extracted from the symbol waveform itselfby techniques well know to those skilled in the art of digital modemdesign. Another alternate solution, well-known to modem designers, is todirectly extract both the clock frequency and symbol phase informationfrom the symbol waveform itself, without reference to the sync orsubcarrier in the video waveform.

Seventh, broadcast systems typically have higher bandwidth than the NTSCchannel of 4.2 Mhz, and more symbols per line may be used to convey moredata. However, this additional bandwidth may be used by creating asecond modulation spectrum and frequency-shifting it up above the firstmodulation spectrum. This additional channel may be used to carry anytypes of data, but since it may be less robust than the first modulationspectrum, it might be used to carry augmentation data instead. FIG. 5shows this frequency split as appearing at 4.2 Mhz, but another splitpoint may be used as well, e.g., where “x” is 7 Mhz.

Although various embodiments which incorporate the teachings of thepresent invention have been shown and described in detail herein, thoseskilled in the art can readily devise many other varied embodiments thatstill incorporate these teachings.

What is claimed is:
 1. A method for converting an input digital signalinto a format that allows the input digital signal to be carried over ananalog system having an analog infrastructure, said method comprisingthe steps of: (a) acquiring the input digital signal, wherein said inputdigital signal represents compressed digital video data; and (b)modulating the input digital signal into an active video portion of ananalog signal, wherein said active video portion of said analog signalonly carries said modulated input digital signal.
 2. The method of claim1, further comprising the step of: (c) distributing said analog signalover the analog infrastructure.
 3. The method of claim 1, wherein saidanalog signal is a television signal.
 4. The method of claim 3, whereinsaid analog signal is a National Television Systems Committee (NTSC)compliant signal.
 5. The method of claim 3, wherein said analog signalis a Phase Alternation by Line (PAL) compliant signal.
 6. The method ofclaim 3, wherein said analog signal is a (Sequential Technique andMemory Storage) SECAM compliant signal.
 7. The method of claim 1,wherein said input digital signal is an Advanced Television SystemsCommittee (ATSC) compliant signal.
 8. The method of claim 1, whereinsaid input digital signal is a Digital Video Broadcasting (DVB)compliant signal.
 9. The method of claim 1, wherein said modulating step(b) comprises the steps of: (b1) organizing the input digital signalinto a plurality of symbols; and (b2) modulating said plurality ofsymbols into said analog signal in accordance with a symbol clock thatis related to a reference signal of said analog signal.
 10. The methodof claim 9, wherein said reference signal of said analog signal is acolor burst signal.
 11. The method of claim 9, wherein said modulatingstep (b) further comprises the step of: (b3) pausing said modulatingstep (b2) of said plurality of symbols while said reference signal isbeing generated as a portion of said analog signal.
 12. The method ofclaim 1, wherein said modulating step (b) modulates the input digitalsignal using Quadrature Amplitude Modulation (QAM).
 13. The method ofclaim 1, wherein said input digital data selectively comprises referencedata for measuring channel quality.
 14. A method for extracting an inputdigital signal from an analog signal received over an analog systemhaving an analog infrastructure, said method comprising the steps of:(a) acquiring the analog signal; and (b) demodulating an active videoportion of said analog signal to extract the input digital signal,wherein said input digital signal represents compressed digital videodata, wherein said active video portion of said analog signal onlycarries said modulated input digital signal.
 15. The method of claim 14,wherein said analog signal is a television signal.
 16. An apparatus forconverting an input digital signal into a format that allows the inputdigital signal to be carried over an analog system having an analoginfrastructure, said apparatus comprising: a buffer for storing theinput digital signal, wherein said input digital signal representscompressed digital video data; and a modulator, coupled to said buffer,for modulating the input digital signal into an active video portion ofan analog signal, wherein said active video portion of said analogsignal only carries said modulated input digital signal.
 17. Theapparatus of claim 16, wherein said modulator organizes the inputdigital signal into a plurality of symbols and modulates said pluralityof symbols into said analog signal in accordance with a symbol clockthat is related to a reference signal of said analog signal.
 18. Theapparatus of claim 17, further comprising: a controller, coupled to saidbuffer, for controlling said buffer to pause the forwarding of saidplurality of symbols to said modulator, while said reference signal isbeing generated as a portion of said analog signal.
 19. An apparatus forextracting an input digital signal from an analog signal received overan analog system having an analog infrastructure, said apparatuscomprising: a demodulator for demodulating an active video portion ofsaid analog signal to extract the input digital signal, wherein saidinput digital signal represents compressed digital video data, whereinsaid active video portion of said analog signal only carries saidmodulated input digital signal; and a buffer, coupled to saiddemodulator, for storing the input digital signal.
 20. Acomputer-readable medium having stores thereon a plurality ofinstructions, the plurality of instructions including instructionswhich, when executed by a processor, cause the processor to perform thesteps comprising of: (a) acquiring the input digital signal, whereinsaid input digital signal represents compressed digital video data; and(b) modulating the input digital signal into an active video portion ofan analog signal, wherein said active video portion of said analogsignal only carries said modulated input digital signal.
 21. Acomputer-readable medium having stored thereon a plurality ofinstructions, the plurality of instructions including instructionswhich, when executed by a processor, cause the processor to perform thesteps comprising of: (a) acquiring the analog signal; and (b)demodulating an active video portion of said analog signal to extractthe input digital signal, wherein said input digital signal representscompressed digital video data, wherein said active video portion of saidanalog signal only carries said modulated input digital signal.